1. Technical Field
The present invention relates to an interconnect for a solid oxide fuel cell and a method for manufacturing the same.
2. Description of the Related Art
Since oil currently widely used as an energy source is limited in reserves thereof, alternative energy substituting for the petroleum is a major national and social issue. For example, interests on electricity generation using solar heat, tides, and wind, or fuel cells, rather than fossil fuels have been growing.
The above fuel cell generates electricity by using a reverse reaction of an electrolysis reaction of water, and applies technology of converting hydrogen contained a hydrocarbon based material such as natural gas, coal gas, methanol, or the like, and oxygen in the air into electricity energy through an electrochemical reaction.
Unlike the existing generation technology including several procedures such as fuel combustion, steam generation, turbine driving, generators driving, and the like, the fuel cell has no combustion procedure or driving apparatus and thus has advantages of providing high efficiency, scarcely exhausting air pollutants such as SOx, NOx, and the like, generating a small amount of carbon dioxide, and barely generating noise or vibration.
There are many various kinds of fuel cells, for example, a phosphoric acid fuel cell (PAFC), an alkaline fuel cell (AFC), a polymer electrolyte membrane fuel cell (PEMFC), a direct methanol fuel cell (DMFC), a solid oxide fuel cell (SOFC), and the like.
Among the above fuel cells, the solid oxide fuel cell (SOFC) have several advantages in that an overvoltage based on activation polarization is low and irreversible loss is small, resulting in high generation efficiency; various fuels are usable without a modifier, for example, carbon or hydrocarbon based fuel as well as hydrogen is usable and thus fuel selective width is wide; and the reaction rate at the electrode is high and thus noble metal catalysts are not needed. In addition, since very high heat is generated during the reaction, high-temperature heat may be used for modifying fuel or as an industrial or refrigerating energy source.
This solid oxide fuel cell (SOFC) has an electrode reaction such as the following reaction formulaFuel electrode: H2+O2−→H2O+2e−CO+O2−→CO2+2e−Air electrode: O2+4e−→2O2−Total reaction: H2+CO+O2→H2O+CO2  [Reaction Formula]
In the fuel cell operating according to the above reaction formula, electrons reach an air electrode passing through an external circuit, and at the same time, oxygen ions generated from the air electrode reach a fuel electrode through an electrolyte, and thus at the fuel electrode, hydrogen or CO combines with oxygen ions to generate electrons and water or CO2.
Meanwhile, the above solid oxide fuel cell has a unit cell composed of a fuel electrode, an electrolyte, and an air electrode, and interconnects disposed above and below the unit cell, to collect electricity generated from the unit cell and supply fuel and air to the fuel electrode and the air electrode, respectively. The interconnect is surface-connected with the unit cell, and thus directly receives heat of the unit cell.
However, the existing interconnect is formed of a metal material, and thus may be easily oxidized in a high-temperature oxidation ambience to form an oxide film, and a chrome component inside the interconnect may move the electrodes or the electrolyte, which causes a secondary phase together with the components of the electrodes or the electrolyte. The oxide film deteriorates electrical conductivity of the interconnect, resulting in decreasing electricity collecting efficiency, and in particular, the formation of the secondary phase due to the chrome component significantly deteriorates performances of the electrodes and the electrolyte.
In order to solve the foregoing problems, an oxidation-resistant coating layer or a film layer may be formed on the interconnect. However, the oxidation-resistant coating layer or the film layer has a difference in coefficient of thermal expansion from the inner interconnect, and thus causes delamination from the interconnect within a short time. Moreover, the oxidation-resistant coating layer or the film layer also has a difference in coefficient of thermal expansion from a glass sealing agent on a lateral surface, which causes sealing problems.
For example, U.S. Pat. No. 8,173,328 (Interconnects for Solid Oxide Fuel Cells and Ferritic Stainless Steels Adapted for Use with Solid Oxide Fuel Cells) discloses that oxidation is prevented through surface treatment of a metal interconnect having holes and gas channels, but this technology cannot completely prevent high-temperature oxidation of a metal material and cannot solve the problem that the chrome component in the terrific stainless steel reacts with the components of the fuel cell, which deteriorates durability of the cell.